Actaully - they don't. I spent some time working with Oracle at a major installation where they could not come up with a good enough transactional fail secarios because a single mainframe was being replaced by a multiple DB node distributed system.

Oddly - the project failed...

Just because a large company says they do something - does not mean they actually do. Distributed Transcation as said by RDBMS companies actually is 'Distributed as long as nothing goes wrong'.